Diagenetic formation of uranium-silica polymers in lake sediments over 3,300 years
Résumé
The storage of large volumes of uranium (U) mine waste and consequent contaminated sites is a widespread environmental challenge. Essential for their management is the proper understanding of U immobilization mechanisms in reducing environments. In particular, the long-term behavior of recently discovered non-crystalline U(IV) species in (sub-)surface conditions is poorly documented, which limits our ability to predict U long-term geochemical reactivity. Here, we report direct evidence for the evolution of U speciation over 3,300 years in naturally highly U-enriched sediments from Lake Nègre (Mercantour Massif, Mediterranean Alps, France) by combining U isotopic data (δ238U and (234U/238U)) with U L3-edge X-Ray Absorption Spectroscopy. Constant isotopic ratios over the entire sediment core indicate stable U sources and accumulation modes, allowing for determination of the sole impact of ageing on U speciation. We demonstrate that non-crystalline U(IV) associated to organic matter polymerizes to a nano-crystalline coffinite (UIVSiO4·nH2O)-like phase in less than 700 years. This transformation does not significantly impact U remobilization with respect to its non-crystalline precursor. Sustainable reducing conditions are thus required to allow long-term U sequestration in widely encountered organic and silica-rich environments.
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